It is sought more particularly here below in this document to describe problems existing in the field of marine seismic prospection. The invention of course is not limited tot his particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
In marine seismic exploration, the operations of acquiring seismic data on site conventionally use networks of sensors distributed along cables in order to form linear acoustic antennas, also referred to as “streamers” or “seismic streamers” or “seismic cables”. The seismic streamers are towed through water behind a seismic vessel at a water depth that can be more or less deep.
In the following description, a seismic cable is understood to be a seismic streamer which can be maintained at a selected depth under the sea surface or a cable lying on the sea bed, for example in an Ocean Bottom configuration, also known as Ocean Bottom Cable (OBC) for seabed acquisition.
A c marine seismic method is usually based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the sensors (e.g. hydrophones, geophones, accelerometers or the like) distributed over the length of the seismic cables. These acoustic signals are processed and transmitted through the telemetry from the seismic cables to a central unit onboard the seismic vessel, where they are stored.
As shown in FIG. 1, a network of seismic cables 100 to 104 is towed by a seismic vessel 115. A seismic cable 100 generally comprises a plurality of controllers 20, such as telemetry modules, arranged along the seismic cable 100. Part of cable comprised between two controllers 20 forms a portion of cable. Each portion of cable is itself divided into a plurality of cable sections comprising each a plurality of seismic sensors, such as hydrophones or geophones or accelerometers, arranged along the section and adapted for detecting acoustic signals. The seismic cable 100 shown in FIG. 1 comprises, for example, two portions of cable P1 and P2 each comprising three cable sections S1 to S3 comprising seismic sensors for data acquisition. The seismic sensors are referenced 10 in FIG. 2, which illustrates in detail the block referenced A in FIG. 1, i.e. a portion P2 of the seismic cable referenced 100. Each cable section S1 to S3 may further comprise a plurality of nodes (not shown on the figures) distributed along the cable at intervals that are not necessarily regular.
These nodes are connected to the controllers via electrical wires (not shown on figures). More precisely, all the nodes are provided serially along the electrical wires from the head end of the seismic cable until the tail end of the seismic cable, with the controllers being distributed between groups of nodes on the electrical wires. A node is adapted for collecting seismic data issued from a given associated set of sensors 10 and to digitize them before sending them, via the controllers 20, to the central unit situated on the seismic vessel.
Controllers are assembled in series along the seismic cable and each associated with at least one of the nodes, each controller providing power supply and synchronization of the nodes wherewith it is associated. More precisely, the controllers concentrate the data issued from a plurality of nodes. Then, the controllers direct the concentrated data received from the nodes and the sensors upon data transmission lines (such as telemetry line 40) for carrying data from or towards the controllers 20, and to route said data towards the recorder vessel 115.
We will now explain in more details which functions are performed by each node and each controller.
Each node is made up of four basic components:                a sensing unit;        a processing unit;        a transceiver unit; and        a power unit.        
Sensing units are usually composed of two subunits: sensors and analog-to-digital converters (ADCs).
The analog signals produced by the sensors based on the observed phenomenon are converted to digital signals by the ADC, and then fed into the processing unit. The processing unit, which is generally associated with a small storage unit for buffering, manages the procedures that make the nodes collaborate with the other nodes to carry out the assigned sensing tasks. A transceiver unit connects the node to the network.
If sensors are analog sensors (such as geophone or hydrophone), each node performs the analog to digital conversion of the signal from sensors. If sensors are digital (sensors are micro-machined accelerometer for example), no conversion is performed by the corresponding node. Then, these data are sent to a central data processing unit onboard the recorder vessel 115 via the network of data transmission lines. More precisely, each node has the capabilities to collect data and route data back to a controller 20. The data are conventionally sent from the nodes to the central processing unit via controllers 20.
Each controller performs different functions, including:                power supply of the nodes via an high voltage rails;        synchronization of the data;        data retrieval from the nodes through electrical wires;        local storage for buffering of the seismic data;        data routing through the data transmission lines (such as optical telemetry line 40) towards the recorder vessel 115;        interface with the recorder vessel 115 (processing of commands received from the recorder vessel 115);        pre-processing of the data coming from the nodes.        
The nodes and controllers are thus devised to only perform signal processing functions. In other words, the nodes and controllers progressively return the seismic data to the central processing unit.
The power supplying lines supply the controllers and the sensors with a high voltage (such as a voltage about 300V-1000V), so as to limit the level of current in said power supplying lines.
A module of each controller converts the High Voltage received from a power supplying line to a Low Voltage to power the seismic sensors bi-directionally via the power lines. As already explained, the controllers also retrieve and process data from the seismic sensors connected on each side, via electrical wires, and operate interface between the sensors and the data transmission lines 40 by directing the data received from the sensors on the data transmission lines 40.
In the example of FIG. 2, each cable section S1 to S3 has a connector on each of its ends, namely: connectors O11, O12 for the section S1, connectors O21, O22 for the section S2 and connectors O31, O32 for the section S3. Each controller 201 and 202 has a connector on each of its ends, namely: connectors Om1, Om2 for the controller 201 and connectors Os1, Os2 for the controller 202. A connector of a given section is adapted to connect a connector of a section adjacent to said given section, or a connector of a controller adjacent to said given section, so as to allow an electrical and/or optical interconnection when connected.
FIG. 2 illustrates the configuration of a common seismic cable which comprises:                a data transmission line 40 (also called telemetry line) extending along the seismic cable 100 for carrying data from or towards the controllers 20,        a high voltage electrical power supply line 50 extending along the seismic cable 100 for supplying power to the controllers 20 arranged on the cable 100.        
The data transmission line 40 is generally consisted of a set of electrical copper wires for carrying electrical signals and/or a set of optical fibers for carrying optical signals from or towards the controllers 20. The data conveyed by the transmission line 40 belongs to the group comprising: seismic data issued from sensors 10 and control data issued from nodes (for example results of tests on sensors), control data issued from a master controller to a slave controller and/or from a slave controller to a master controller (for example leakage of a controller, overconsumption of a controller).
The electrical power supply line 50 is adapted for supplying in cascade pairs of master and slave controllers 20 on portions of the electrical power supply line 50. Generally speaking, a master controller of a pair of master and slave controllers is responsible for monitoring the portion of electrical power supply line comprised between this pair of master and slave controllers. For example, on the seismic cable portion P2 illustrated in FIG. 2, the master controller 201 is responsible for managing the electrical power supply to the slave controller 202. The master controller 201 also monitors the electrical power supply of nodes arranged along the cable portion P2.
It should be noted that the master controller manages the power supply to the slave controller which can be placed before (i.e. vessel side, as illustrated on FIG. 2) or after (i.e. seismic cable end side), and not necessary immediately adjacent to the master controller. In the example of FIG. 2, the slave controller 202 is placed immediately adjacent to the master controller 201, so that the power supply is carried out for two immediate successive master and slave controllers. This is, of course, a particular embodiment of implementation.
In marine seismic exploration, due to the length of the seismic cable, the controllers are supplied with high voltage from the seismic vessel (typically comprised between 300 and 900 V) via the electrical power supply line 50, which poses problems of safety.
In order to prevent possible risks of electrocution or physical damage when the seismic cable is damaged or is open, each portion of seismic cable 100 comprised between a master controller 201 and the slave controller 202 is equipped with a safety loop that allows to stop the high voltage supply if necessary. As shown on FIG. 2, the safety loop SL is composed of a pair of two electrical copper wires 61, 62 comprised between the master 201 and slave 202 controllers. The pair of electrical copper wires 61 and 62 are connected via an impedance element 63, of specific impedance value, which is comprised in the slave controller 202. A measurement unit 64 placed within the master controller 201 is adapted for performing an impedance measurement.
If the impedance measured by the measurement unit 64 is equal to said specific impedance value 63, this means that the safety loop is closed. In that case, the master controller 201 can propagates (or continues to propagate) the high voltage to the slave controller 202 via the portion of the electrical power supply line 50 comprised between these two controllers.
If the impedance tends towards infinity, this means that the safety loop is open and so the seismic cable 100 is open or damaged. In that case, the master controller 201 must stop (or not launch) the electrical supply sent to the slave controller 202 to allow interventions (seismic cable retrieving, fault or damage location, change of deficient portion or elements of the cable, etc.).
However, this known solution has a number of drawbacks. The use of a safety loop based on electrical impedance measurement requires two additional electrical copper lines per electrical power supply wire (a line electrical power supply line typically comprises a set of two electrical power supply wires), which has a significant impact on the overall weight and size of the seismic cable, but also on the size of the connectors.